add mixin for some NSApplication constants
use calloc with ffi
libffi in 10.15+ has different usage (presumably for arm64)
arm64 has objc_sendMsg only
add missing headers
no more mach_override
remove unwanted objs
add missing function definitions
fiddle around with build settings
disable mach_override and grab the interpreter lock when performing respondsToSelector on a pike object
disable use of mach_override
fix some more leaks identified by the leaks tool

note that the free_svalue leak may not be an actual leak.
fix a bunch of memory leaks, correct Mapping wrapping caused by an "implicit" function definition
fixes for ffi calls on method dispatch: float param and return values in particular
more fixes for objective c 2
add option to skip stub building, and add another SDK location